The anti-Semitic truck driver accused of gunning down 11 people at a Pittsburgh synagogue pleaded not guilty Thursday to federal charges that could put him on death row, as funerals for the victims of the worst atrocity perpetrated against Jews in American history continued for a third day.

Robert Bowers, 46, was arraigned one day after a grand jury issued a 44-count indictment charging him with murder, hate crimes, obstructing the practice of religion and other crimes. It was his second brief appearance in a federal courtroom since the weekend massacre at Tree of Life synagogue in Pittsburgh's Squirrel Hill neighborhood.

Here's what we know about Bowers and the events of Saturday, Oct. 27.

The shooting

Bob Jones, the FBI special agent in charge of the Pittsburgh field office, said 46-year-old Bowers was armed with what appeared to be an assault rifle and three handguns and is believed to have acted alone. Law enforcement sources tell ABC that investigators recovered a total of 10 firearms from the scene and at Bowers' home, all of which were purchased legally.

Two Pittsburgh police officers came into contact with Bowers as Bowers attempted to leave the synagogue, according to a criminal complaint. After a shootout that left the officers wounded -- one was shot in the hand and the other hit by shrapnel -- Bowers retreated into the synagogue.

Rabbi Alvin Berkun, the synagogue's rabbi emeritus, told ABC News he had heard that the gunman at one point was in his office on the third floor of the synagogue. A shootout ensued with SWAT officers on the third floor, the criminal complaint said, leaving two additional officers wounded.

Bowers was eventually taken into custody alive after spending approximately 20 minutes in the synagogue, according to Pittsburgh Public Safety Director Wendell Hissrich.

Local, state and federal officials hold a news conference on the Pittsburgh synagogue shooting on October 27, 2018.

Bowers was transported to a local hospital in fair condition with multiple gunshot wounds. Investigators do not believe his gunshot wounds were self-inflicted.

The shooting left at least 11 people dead. Pennsylvania Attorney General Josh Shapiro said the gunman "claimed innocent lives" at a baby-naming ceremony, though officials said no children are among the dead. At least six people were injured including four police officers. The officers' injuries are not life-threatening, but the others sustained more serious injuries. Hissrich described the crime scene as "horrific" and "one of the worst I've seen."

Investigators are still working to determine the motive behind the shooting, though Jones did say the victims were "targeted simply because of their faith." The shooting is being investigated as a hate crime.

Bowers was not known to law enforcement prior to Saturday's incident. He told a SWAT officer that he "wanted all Jews to die and also that they (Jews) were committing genocide to his people," a criminal complaint alleges.

Social media posts

The social media site Gab.com said Bowers had a profile on their website. A cached version of the account believed to belong to Bowers showed posts that are anti-Semitic and anti-refugee in nature and that are also critical of President Trump.

Before the shooting, a new message was posted to the account: "HIAS likes to bring invaders in that kill our people. I can't sit by and watch my people get slaughtered. Screw your optics, I'm going in."

HIAS is a nonprofit group that helps refugees around the world find safety and freedom. The organization said it is guided by Jewish values and history.

President and CEO Mark Hetfield told the Associated Press he wasn't aware of the shooter's "obsession with HIAS until this morning."

Gab denounced the shooting in a statement, adding that it suspended the alleged gunman's account, backed up the content and notified the FBI. Gab, which is popular with far-right extremists, says its mission is to defend free expression and individual liberty online for all people.

ABC News has not been able to independently verify the social media accounts attributed to Bowers. A counterterrorism official briefed on the probe confirmed to ABC News that officials are looking at the Gab account and other social media platforms believed to be linked to Bowers.